1. Field of the Invention
The invention relates to waveguides in general and more particularly to heat assisted recording. It also relates to near field microscopy and to nanoscale photolithography.
2. Description of the Related Art
Heat-assisted magnetic recording (HAMR) involves heating a spot on the surface of a magnetic recording medium to reduce its coercivity sufficiently so that it can be magnetically recorded by an applied recording field. The advantage of this technique is that the coercivity of the media at ambient can be significantly increased, thereby improving thermal stability of the recorded data even for very small bit cells. One of the difficulties with the technique is finding a method to heat just the small area which is to be recorded. Heating with laser light as is done in magneto-optic recording is the most promising approach, but the difficulty with this is that at the current storage densities contemplated for HAMR, the spot to be heated is ˜25 nm in diameter, which is fifty times smaller than the wavelength of useful semiconductor lasers. The so-called diffraction limit in optics is the smallest dimension to which a light beam can be focused. The diffraction limit in three dimensions is given by the equation
                    d        =                              0.6            ⁢            λ                                n            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                                              (        1        )            
where d is the spot diameter, λ is the wavelength of the light in free space, n is the refractive index of the medium in which the light is focused, and θ is the maximum angle of focused light rays from the central axis of the lens. The factor λ/n is the wavelength of the light within the medium in which the light is focused. The spot diameter is directly proportional to the wavelength of the light within this medium. The minimum focused spot diameter in the classical diffraction limit in air is ˜λ/2, which is much too large to be useful for HAMR.
When light is incident upon a small circular aperture, it is well-known in classical optics that the amount of power transmitted through the aperture scales as the ratio of the aperture to the wavelength raised to the fourth power. For an aperture with a ˜25 nm diameter at a visible light wavelength of 500 nm, a typical transmission efficiency is about 10−6. This throughput is orders of magnitude too small to be practical for HAMR.
In order to circumvent this problem, a “C”-shaped aperture in an opaque metal film presents a possible approach to increasing the amount of power transmitted while keeping the spot size at the dimensions required. This design 10 is illustrated in FIG. 1. It comprises a “C”-shaped aperture 30 located within a conductive body 28. Because this structure requires some thickness to be opaque to incident light, the structure is as also sometimes referred to as a ridge waveguide. A “C”-shaped ridge waveguide has a propagating mode for light polarized perpendicular to the long dimension 22 of the waveguide cross section at a wavelength that is approximately equal to the sum of the length L of the long dimension 22 and twice the height H of the short dimension 16. However, the field density is concentrated in the region between the ridge 12 and the wall 29 opposite.
A calculation of the electric field at the bottom surface of the waveguide using finite-difference time domain (“FDTD”) analysis indicates that there is a large field amplitude in the small region near the tip 26 of the ridge 12 in the waveguide 10. As a result, this structure appears suited as a means to efficiently transport optical energy into a small, sub-wavelength spot at the output end of the waveguide, at least for optical recording.
However, for the C-shaped aperture to be effective for HAMR applications, the energy must be also be efficiently absorbed by the media below the waveguide in a tightly confined spot. FIG. 2 is a contour FDTD plot of the efficiency of power absorption in the media when a typical media surface is spaced just below the waveguide. The FDTD calculation results shown in FIG. 2 are for a wavelength of 900 nm with respect to a gold structure illustrated in FIG. 1 of the following dimensions: The height H 16 of the aperture 30 is 60 nm, the length L 22 of the aperture 30 is 240 nm, the height h 20 of the ridge 12 is 36 nm, and the width w 18 of the ridge 12 is 24 nm. The index of refraction for gold is 0.17+i(5.95). Below the ridge waveguide is an air gap of 10 nm, and then the recording medium which consists of a 10 nm layer of cobalt followed by an 80 nm layer of iron. The index of refraction for the cobalt is 2.65+i(5.16). The index of refraction for iron is 3.12+i(3.87). A plane wave is incident normally from the top of the structure.
In FIG. 2, a contour plot is shown of the optical power that is dissipated within the recording medium below the ridge waveguide. The bright region 32 represents a “hot” spot where the highest density of power dissipation occurs. The dark regions 34 represent colder spots. The black lines 31 and 33 are the contours at the full width half maximum (FWHM) level. The dashed line corresponds to waveguide aperture 30.
FIG. 2 indicates that the optical power is very poorly confined by the waveguide/media combined structure. The electric field wraps around the sides and bottom of the ridge 12 in the waveguide 10 and spreads out into “wings” underneath the waveguide. The horizontal distance between FWHM contour lines 31 at hot spot 32 is about 70 nm at its narrowest part even though the ridge width is only 24 nm. The vertical distance between FWHM contour lines 33 at hot spot 32 is >>100 nm long even though the ridge 12 height is 36 nm. This elongation of the hot spot 32 is a result of the confinement of the field between the bottom of the ridge of the waveguide 10 and the media surface. According to this FDTD analysis, this waveguide structure is not useful for generating a small optical hot spot for heat assisted recording.